1. Field of the Invention
The present invention relates generally to a system for monitoring a beacon signal, and more specifically to an improved receiver for identifying and tracking a signal generated by a portable transmitter. If the transmitter is placed with currency or other valuables, the invention can be used to facilitate the location and pursuit of the transmitter and apprehend a would-be thief. While the invention will be illustrated by a system for the apprehension of the perpetrators of a theft, other applications of the invention will immediately suggest themselves, including the location and tracking of persons, vehicles and other portable objects.
2. Description of the Art
As stated above, the present invention may be used to facilitate and improve the deterrence of crime and the apprehension of criminals. A radio frequency transmitter is initially secreted in a packet of currency or other valuables. The packet may comprise a packet of real currency which has been modified so as to provide an interior recess wherein is located the transmitter. Alternatively, the packet may comprise simulated currency similarly configured so as to provide an internal recess for the transmitter. In other cases it may be preferable to combine real currency with simulated currency to make up the packet.
The battery powered transmitter may be energized through a switch which is in a normally open state when the currency packet is located in the currency drawer or other storage compartment. The act of removing the currency packet from the storage location causes this switch to close, thereby energizing the transmitter to transmit at a predetermined radio frequency.
Receivers for monitoring and tracking a portable transmitter have long been known in the prior art. Prior methods used for tracking a portable transmitter have used a dual channel radio frequency (RF) receiver with a two antenna system used as an interferometer to measure the phase difference of the received signal to determine the angle of arrival of the signal. FIG. 1 shows such a prior art system. Antennas 1A and 1B received transmitted data, which is input into RF circuitries 2 and 3. The information is processed and input into IF circuitries 4 and 5. The outputs of IF circuitries 4 and 5 are used by angle detector circuitry 6, which uses phase differences to determine the angle between the transmitter and receiver. This data is output as direction information 8A. Signal detector circuitry identifies the transmitting signal and generates signal alert information 8C. Signal level information 8B is also generated by IF circuitry 5. This angle information could then be used to determine the direction from the receiver to the transmitting signal. Examples of such prior art are shown in U.S. Pat. Nos. 4,021,807, 4,001,828 and 4,023,176, which are incorporated herein by reference.
While useful, this prior art system has several major disadvantages. In particular, the system shown in FIG. 1 only provides unambiguous direction information for an are of 180 degrees. Thus, the system requires manual operation to switch to a rear antenna 1C to determine if the signal is in front or rear of the receiver. Moreover, the system shown in FIG. 1 requires two complete RF receiver channels. In addition, the system requires high dynamic-range phase tracking within the narrow intermediate frequency (IF) bandwidth, leading to increased noise and error.
An improvement on this system is shown in FIG. 2. FIG. 2 shows a single RF channel receiver which can process the information from an electronically scanned, three-element antenna array. Information from antennas 10A, 10B, and 10C is selected by antenna switching circuitry 11. This information is then processed by RF circuitry 12 and IF circuitry 13. FM detector circuitry 14 provides input to the angle detector circuitry 15, which outputs direction information. Antenna waveform generator circuitry 18 provides a waveform to antenna switching circuitry 11. AM detector circuitry receives the output from IF circuitry 13 and provides signal level information 19B and signal alert information 19C. Automatic gain control 17 is coupled to the RF circuitry 12. A receiver design which uses the method of operation shown in FIG. 2 is now being used by ProNet, Inc. The direction information 19A is provided by a frequency modulation (FM) demodulator which provides the detected antenna modulation that is then compared to the reference signal used to generate the antenna switching. The phase difference in this comparison is proportional to the angle of arrival of the signal.
This system has several advantages over the system shown in FIG. 1. For example, the system requires only one RF receiver channel. In addition, the system shown in FIG. 2 does not require high dynamic range phase tracking of two RF channels. Also, the newer system provides an unambiguous 360 degree continuous direction measurement.
The system shown in FIG. 2, however, has several disadvantages. In particular, the antenna switching circuitry 11 results in a spectrum spreading of the input signal based on the waveforms used in the antenna switching from antenna generating circuitry 18. The system shown in FIG. 2 uses a square pulse antenna switch control signal, resulting in a sin(x)/x spectrum that causes out-of-band signals to be folded into the operating band. This results in out-of-band signal energy being folded into the band of operation and causes interference. Another disadvantage of the system shown in FIG. 2 is that it uses a wide band phase-locked loop (PLL) demodulator to detect the antenna modulation, resulting in low sensitivity to and capture of signals in the IF band of the receiver. A wide bandwidth is thus required to allow for the frequency uncertainty of the signal.
Other disadvantages of the system shown in FIG. 2 include the following:
the angle detector circuitry 15 uses a set/reset pulse phase comparator to measure the antenna phase angle, resulting in a higher signal-to-noise ratio required for a given phase measurement accuracy; PA1 the AM detector circuitry 16 uses noncoherent amplitude modulation (AM) detection to detect the transmitter signal. This results in a higher required signal-to-noise than would a coherent AM detector; PA1 the system requires a slow responding automatic gain control (AGC) 17 to operate over the dynamic range of operation for AM detection. This causes intermittent bursts of inference to capture the AGC; and PA1 the system uses a single 90 Hz tone modulation to activate the signal alert information. This is subject to false alarms in identifying the transmitter signal due to many types of electromagnetic interference (EMI) and interfering signals which appear to have the 90 Hz modulation.
It is therefore an object of the present invention to reduce spectrum-spreading and resulting interference in the receiver.
It is a further object of the present invention to increase the sensitivity of the receiver to RF signals.
It is yet a further object of the present invention to decrease the signal-to-noise ratio required for a given phase measurement accuracy in a receiver.
It is a further object of the present invention to decrease the reliance on AGC in the receiver.
It is yet a further object of the present invention to reduce the number of false alarms in identifying the transmitter signal.
It is a further object of the present invention to increase the operating range of the receiver.
Other objects of the invention are apparent from the summary, drawings and detailed description below.